1. Field of the Invention
This invention relates to a process for treatment of water vapor which is generated in a step of concentrating an aqueous urea solution containing small amounts of ammonia and carbon dioxide.
2. Description of the Prior Art
The production of crystalline urea from carbon dioxide and ammonia has been heretofore carried out by a process wherein the carbon dioxide and ammonia are reacted with each other under urea-systhesis temperatures and pressures to obtain a reaction mixture containing urea, unreacted ammonium carbamate and water, and the reaction mixture is then passed through a plurality of unreacted ammonium carbamate-decomposition stages, the pressures of which are stepwise reduced. Such multi-stage decomposition is generally effected by a two-stage high and low pressure decomposition process, a three-stage decomposition process involving two high pressure stages and one low pressure stage, or an one- or two-stage high pressure and/or low pressure decomposition process which is carried out subsequent to stripping with ammonia or carbon dioxide under a pressure substantially equal to the urea-synthesis pressure, and a separation step involving flashing under normal or reduced pressure is generally provided subsequent to a low pressure decomposition to separate the unreacted ammonium carbamate from the reaction mixture in the form of a mixed gas of ammonia and carbon dioxide. Then the resulting urea solution which contains small amounts of ammonia and carbon dioxide is subjected to concentration, preferably under reduced pressure, to obtain crystalline urea. In the process, water vapor which is generated in the concentration step contains a small amount of a mist of the aqueous urea solution, together with ammonia and carbon dioxide. Accordingly, discharging the condensate formed by condensing this water vapor as it is results not only in water pollution but also in loss of urea and ammonia. In order to avoid these disadvantages, the water vapor has heretofore been condensed by indirect cooling to form a condensate containing urea, ammonia and carbon dioxide, the thus formed condensate is subjected to rectification for separating therefrom a mixed gas of ammonia, carbon dioxide and water vapor, and the mixed gas is recovered to feed back into the urea synthesis step. However, since the waste water discharged from the rectification step still contains some urea, even though a part of the urea is hydrolyzed during the rectification into ammonia and carbon dioxide which are collected together with the above-mentioned mixed gas, discharging this waste water from the urea plant still presents problems of water pollution as well as of loss of urea.